Eyepiece calculations and technical details 691 The exit pupil

The exit pupil is the bundle of light rays emerging from the eyepiece into the observer's eye (Figure 5.1, p. 60). You can see it as a bright disk viewed from a few inches away (Figure 6.4). Its size depends on aperture and magnification:

Telescope aperture

Diameter of exit pupil =- -


Telescope aperture

Diameter of exit pupil =- -


Figure 6.4. The exit pupil is the bright spot that you see in the middle of the eyepiece.

For example, an 8-inch (20-cm) telescope at 100x has an exit pupil diameter of: 20 cm

Higher powers give smaller exit pupils, and any given exit pupil size corresponds to a fixed "power per inch" or "power per cm" of aperture:

Aperture 1

Exit pupil diameter Exit pupil diameter

For example, a 0.1-cm exit pupil corresponds to a magnification of 10 times the aperture in centimeters ("10x per cm").

Exit pupil size can also be found from f -ratio and eyepiece focal length:

Eyepiece focal length

Telescope f -ratio

Eyepiece focal length = Exit pupil diameter x Telescope f-ratio For example, a 32-mm eyepiece gives a 3.2-mm exit pupil with any f /10 telescope.

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